Representation of the visual field in the lateral intraparietal area of macaque monkeys: a quantitative receptive field analysis

The representation of the visual field in the primate lateral intraparietal area (LIP) was examined, using a rapid, computer-driven receptive field (R F) mapping procedure. RF characteristics of single LIP neurons could thus b e measured repeatedly under different behavioral conditions, Here we report data obtained using a standard ocular fixation task during which the anima ls were required to monitor small changes in color of the fixated target. I n a first step, statistical analyses were conducted in order to establish t he experimental limits of the mapping procedure on 171 LIP neurons recorded from three hemispheres of two macaque monkeys. The characteristics of the receptive fields of LIP neurons were analyzed at the single cell and at the population level. Although for many neurons the assumption of a simple two -dimensional gaussian profile with a central area of maximal excitability a t the center and progressively decreasing response strength at the peripher y can represent relatively accurately the spatial structure of the RF, abou t 19% of the cells had a markedly asymmetrical shape. At the population lev el, we observed, in agreement with prior studies, a systematic relation bet ween RF size and eccentricity. However, we also found a more accentuated ov errepresentation of the central visual field than had been previously repor ted and no marked differences between the upper and lower visual representa tion of space. This observation correlates with an extension of the definit ion of LIP from the posterior third of the lateral intraparietal sulcus to most of the middle and posterior thirds. Detailed histological analyses of the recorded hemispheres suggest that there exists, in this newly defined u nitary functional cortical area, a coarse but systematic topographical orga nization in area LIP that supports the distinction between its dorsal and v entral regions, LIPd and LIPv, respectively. Paralleling the physiological data, the central visual field is mostly represented in the middle dorsal r egion and the visual periphery more ventral and posterior. An anteroposteri or gradient from the lower to the upper visual field representations can al so be identified. In conclusion, this study provides the basis for a reliab le mapping method in awake monkeys and a reference for the organization of the properties of the visual space representation in an area LIP extended w ith respect to the previously described LIP and showing a relative emphasis of central visual field.